1. Field of the Invention
The present invention relates in general to tubular hydroforming and more particularly to method and device for forming a tubular work into a shaped hollow product by using hydroforming process. More specifically, the present invention relates to method and device for producing an automotive hollow part, such as front pillar, center pillar, roof rail or the like, by using tubular hydroforming process.
2. Description of the Prior Art
Tubular hydroforming process is a novel process that has recently gained much attention due to its cost-effective application particularly in the automotive industry. As is known, the tubular hydroforming is of a process that includes major steps wherein ends of a tubular work in a net shape die unit are sealed and a hydraulic fluid is pumped in the tubular work and pressurized to deform cross-sections of the work to conform to a cross section of the net shape die. Usually, before the major steps, a pre-forming is made wherein for obtaining a pre-defined shape of the tube that closely resembles the final component (viz., hollow product), a die closing is gradually carried out while receiving a relatively low hydraulic fluid in the work. While, in a so-called bulging process in the tubular hydroforming, axial feed is provided along the longitudinal axis of the tubular work in the net shape die while receiving the hydraulic fluid in the work. When employing this bulging process, a tube wall thinning during the hydroforming process can be reduced.
However, due to the nature of deformation of the material of the tubular work during the hydroforming process, it has been difficult to provide a hydroformed hollow product that gives users satisfaction. In fact, in the pre-forming step, even when aluminum and/or high strength steel tube is used as the tubular work, a crack tends to appear at a portion that has been subjected to a wall thinning during the expansion of the work. Furthermore, in the pre-forming step, a corner portion remote from the center of the work is particularly attacked by such wall thinning. In the bulging process, wall thickening throughout the length of the tubular work is readily made, however wall thickening at a specified or needed portion, such as a corner portion or the like, is not readily made, and thus, reduction in weight of the hydroformed hollow product has not been satisfactorily achieved particularly in the field of automotive industry.
It is therefore an object of the present invention to provide a method for forming a tubular work into a shaped hollow product by using hydroforming process, which method is free of the above-mentioned drawbacks.
It is further an object of the present invention to provide a hydroforming device which is suitable for practically carrying out the method of the present invention.
It is further an object of the present invention to provide a hydroforming method by which a specified or needed portion of a shaped hollow product can be exclusively thickened.
According to the present invention, there is provided a method for forming a tubular work into a shaped hollow product by using hydroforming process. In the method, female and male dies are prepared. The female die has a longitudinally extending cavity which has a polygonal cross section when receiving the male die. The tubular work is placed into the cavity of the female die. The interior of the tubular work is then fed with a hydraulic fluid, and the pressure of the fluid is increased to a given level. The given level is smaller than a critical level that causes a bulging of the tubular work. The male die is then pressed against the tubular work to deform the same while keeping the hydraulic fluid at the given level, thereby forming a shaped hollow product that has a polygonal cross section that conforms to that of the cavity. The pressing work is continued until a circumferential length of the shaped hollow product becomes shorter than that of the tubular work.
According to the present invention, there is further provided a hydroforming device for forming a tubular work into a shaped hollow product by using a hydroforming process. The device comprises a fixed female die having a longitudinally extending cavity, the cavity being sized to receive therein the tubular work; a male die having a work surface, the male die being movably received in the female die in such a manner that the work surface of the male die faces the cavity to cause the cavity to be enclosed and have a polygonal cross section; at least one projection formed on a lateral end of the work surface, the projection having a sloped surface angled relative to the work surface and an actuator which actuates the male die to press against the tubular work.